Device and method for reating a gas/liquid mixture

ABSTRACT

The present invention relates to a device for treating a gas/liquid mixture, comprising: —an upright vessel ( 1 ) with a lower and upper compartment; —an inlet ( 2 ) for admitting the flow of mixture into the lower compartment; —an agglomerating unit ( 10 ) placed between the lower and upper compartment for enlarging the liquid droplets in the mixture; —a lower outlet ( 4 ) for discharging the substantially liquid-containing mixture part from the lower compartment; —an upper outlet ( 5 ) for discharging the substantially gas-containing mixture part from the upper compartment; characterized by —collecting means ( 11 ) for collecting on the downstream side of the agglomerating unit liquid droplets which have broken through the agglomerating unit; —recycling means ( 12 ) for recycling the collected liquid to the lower compartment from the collecting means.

The present invention relates to a device and method for treating agas/liquid mixture, and in particular to the separation of mixtures ofoil, water and/or gas.

In the oil and gas industry separators are known for separating thesupplied mixture of liquid (oil and/or water) and gas into a stream ofsubstantially gas and a stream of substantially liquid. Differentseparators are known for separating such gas/liquid mixtures.

Known from EP 0 195 464 A1 is a separator consisting of an uprightcolumn (upright vessel), a lower part of which is separated from anupper part by a “demister” or “coalescer” of a mesh of wires, alsoreferred to as a “mesh pad”. The lower part forms a lower compartmentinto which is fed the gas/liquid mixture for treating, while optionallybeing pretreated. During infeed a part of the liquid is alreadyseparated from the mixture. This liquid accumulates at the bottom of thelower compartment.

The remaining part of the gas/liquid mixture is then guided through thewire mesh. The liquid droplets in the mixture for guiding through thewire mesh collide with the wires and grow therewith into a liquid layer.If the speed of the supplied gas/liquid mixture is sufficiently low, theliquid from the liquid layer will drop back under the influence of theforce of gravity into the lower compartment and fall into the liquidalready present there. A discharge is connected to the lower compartmentfor draining liquid, while in the upper compartment a discharge isconnected for draining the gas/liquid mixture remaining after thedemister.

The known demisters have a relatively small pressure drop and a highseparating efficiency, wherein the very small liquid droplets can alsobe separated from the gas. At relatively low gas velocities the liquidremoved from the gas showers downward from the demister under theinfluence of gravity. At higher gas velocities some of the liquiddroplets are entrained by the upward flowing gas and carried along inupward direction from the demister. However, because a liquid layer iscreated on top of the demister at higher gas velocities, flooding limitsthe maximum flow rate of the demister in the standard configuration. Ata determined liquid and gas load the mesh will break through, whichmeans that the mesh becomes supersaturated with liquid and a part of theliquid penetrates through the mesh and flows upward on the downstreamside of the mesh. Such a breakthrough results in the separation ofliquid from the gas/liquid mixture being impeded.

In light of the foregoing, the processing capacity of the conventionaldemister remains limited, and a demister suitable for a determinedprocessing capacity is relatively large.

It is an object of the present invention to obviate at least one of theabove stated drawbacks and other drawbacks associated with the prior artseparators, and to provide an improved method and device wherein theliquid treatment capacity as well as the gas treatment capacity can beconsiderably increased.

According to a first aspect, the present invention relates to a devicefor treating a gas/liquid mixture, comprising:

-   -   an upright vessel with a lower and upper compartment;    -   an inlet for admitting the flow of mixture into the lower        compartment;    -   an agglomerating unit placed between the lower and upper        compartment for enlarging the liquid droplets in the mixture;    -   collecting means for collecting on the downstream side of the        agglomerating unit liquid droplets which have broken through the        agglomerating unit;    -   recycling means for recycling the collected liquid to the lower        compartment from the collecting means;    -   a lower outlet for discharging the substantially        liquid-containing mixture part from the lower compartment;    -   an upper outlet for discharging the substantially gas-containing        mixture part from the upper compartment.

The agglomerating unit causes agglomeration of the liquid, i.e.collecting or accumulating of the liquid in relatively large liquiddroplets. The agglomerating unit is preferably embodied in a mesh, forinstance in the form of a number of layers of metal gauze. Other typesof agglomerating units can however also be applied, such as one or morelayers of structured packing or of vanes or vane packs. Any porousmedium with a porosity of 80 to 99.9% is in fact suitable. The meshaccording to the invention is designed to function in a floodedcondition. The liquid permeating through the mesh as a result ofsupersaturation forms a bubbling mass above the downstream agglomeratingunit surface. In order to prevent too much liquid collecting above theagglomerating unit surface and the distribution of the liquid displacingto the further separator still being adversely affected, the liquid isdischarged via the collecting means and recycling means. The determiningfactor for the separating capacity is now no longer the saturation pointof the mesh, but the (larger) capacity of the collecting and dischargingmeans.

There will moreover be an increase in the total separating efficiency,i.e. the efficiency of the agglomerating unit in combination with anoptional separator arranged downstream. The agglomerating unit has theresult that the dimensions of the downstream liquid droplet areenlarged, which increases the separating efficiency of an optionaldownstream separator such as a cyclone separator. The agglomerating unitwill moreover equalize a possibly irregular distribution on the upstreamsurface of the mesh, which increases the separating efficiency of thedownstream cyclone separator.

In a preferred embodiment of the invention the device comprises aseparator arranged in the upper compartment downstream of theagglomerating unit for further separating the mixture into asubstantially liquid-containing mixture part and a substantiallygas-containing mixture part. The separator preferably comprises one ormore cyclone separators, wherein the inflowing mixture is set into arotating movement, whereby a heavy fraction, in which a relatively largeamount of liquid is present, is flung against the outer wall of thecyclone and is discharged via openings in the side wall.

In a particularly advantageous embodiment the separator comprises one ormore axial recycle cyclones, the liquid discharge of which extends fromthe separator to below the liquid level in the lower compartment. Suchaxial recycle cyclones with very high separation efficiency are knownfrom WO 00/25931, the content of which should be deemed as interpolatedherein. Described herein is an installation wherein a number of boxeswith cyclones are arranged above the vessel. These cyclones are axialrecycle cyclones. The installation comprises a liquid discharge conduitor “downcomer” for carrying the liquid to the underside of the vessel.The outlet nozzle of the liquid discharge conduit has to be situatedbelow the liquid level in order to form a liquid trap and to preventbypass of gas.

In order to ensure that the breakthrough of liquid through the mesh isuniform, i.e. that the breakthrough occurs all at once and substantiallyover the whole cross-section of the agglomerating unit, and thedistribution of the mixture downstream of the agglomerating unit isdistributed uniformly over the cross-section of the vessel, in a furtherpreferred embodiment of the invention the agglomerating unit comprises asubstantially horizontally extending layer of mesh and, in an even morepreferred embodiment, the agglomerating unit has a substantiallyconstant layer thickness.

In a preferred embodiment the collecting means and the recycling meanscomprise at least one collecting reservoir extending in theagglomerating unit for collecting the broken-through liquid therein, andone or more discharge conduits extending from the collecting reservoirto below the level of the liquid collected in the lower compartment. Thecollecting and recycling means are integrated into the mesh. Thecollecting reservoir or collecting trough with the discharge conduitruns through the mesh, whereby the downstream side of the mesh isconnected to the upstream side. The exact geometry of the drainingsystem, for instance the number and the form of the collectingreservoirs and recycle conduits, is determined by the liquid and gasload for processing.

In a further preferred embodiment the inlet is moreover connected to apre-treatment unit for carrying out a first liquid/gas separation. Theeffect of the subsequent treatments by the agglomerating unit and theseparator unit is hereby increased. A pre-treatment unit can be formedby a device which is placed on the inlet stub of a separating vessel andwhich is provided with a number of curved blades which uniformly absorbthe moment of the incoming gas-liquid flow. The blades then guide thegas-liquid flow laterally into the lower compartment of the separatingvessel. As a result of this controlled inflow of the gas-liquid mixture,a first part of the liquid will already be separated whereby the liquidload on the agglomerating unit and separator mounted downstream isconsiderably reduced.

The pre-treatment unit preferably comprises one or more inlet cycloneseparators arranged in the lower compartment. With such a pre-treatmentthe impact of the inlet flow can be limited, a part of the liquid canalready be separated and a more uniform distribution of the mixture flowfalling onto the mesh can be brought about, which enhances the overallseparating efficiency.

It has been found that for an efficient discharge of the liquid dropletdownstream of the agglomerating unit, at a relatively small pressureloss as a result of limiting the through-flow surface area due to thepresence of the collecting and discharging means, the collecting meansare arranged over about 15% of the cross-section of the vessel.

A characteristic measure for the breakthrough point defined by thebreakthrough of the liquid is the so-called K-value. The K-value isproportional to the gas velocity multiplied by the root of the ratio ofthe gas density to the density difference of gas-liquid. If the K-valueis high, the conventional mesh becomes supersaturated and will then nolonger function correctly. The K-value is therefore kept low in aconventional mesh. Limitation of the K-value however also entails alimitation of the separating capacity of the device, which in turnresults in relatively large separators. While in the known devicesbreakthrough of the mesh is always prevented by keeping the K-value low,in practice less than about 0.1, it is precisely the case according tothe invention that a break through the mesh has to occur. The meshapplied according to the invention is therefore embodied to allow thesupplied liquid to break through from a minimum K-value of about 0.1.

Another aspect of the present invention relates to a method for treatinga gas/liquid mixture in an upright vessel with a lower and uppercompartment, comprising of:

-   -   feeding the gas/liquid flow into the lower compartment of the        column;    -   guiding the gas/liquid flow through an agglomerating unit        arranged between the lower and upper compartment at high speed        such that liquid breaks through to a position beyond the        downstream surface of the agglomerating unit;    -   collecting the broken-through liquid;    -   recycling the collected liquid to the lower compartment;    -   discharging the mixture from the lower compartment;    -   discharging the mixture from the upper compartment.

The breaking of the liquid through a mesh takes place at a K-value of atleast 0.1. The supplied gas/liquid mixture must therefore have a K-valueof at least 0.1 to bring about a break through the mesh.

An advantageous embodiment of the method further comprises of furtherseparating the mixture in the upper compartment by guiding the mixturethrough one or more cyclone separators and carrying the separated liquidpart to the lower compartment and the separated gas part to the upperoutlet, preferably combined with pre-treating the introduced gas/liquidflow for separating a part of the liquid from the gas/liquid mixture.

Further advantages, features and details of the present invention willfollow from the description of two preferred embodiments thereof.Reference is made in the description to the figures, in which:

FIG. 1 shows a partly schematic perspective view of an installation forseparating a gas/liquid mixture, wherein a preferred embodiment of thedevice according to the present invention is applied;

FIG. 2 shows a top view in detail of the preferred embodiment of FIG. 1;

FIG. 3 shows a cross-section through a mesh according to the invention;

FIG. 4 is a top view of a further embodiment of a mesh according to theinvention;

FIG. 5 is a partly cut-away perspective view of detail II of FIG. 1.

FIG. 1 shows a separating vessel (separating column) 1 for separating asupplied gas/liquid mixture, such as natural gas mixed with (saltsea-)water, into a substantially gas-containing fraction, also referredto as light fraction, and a substantially liquid-containing fraction(water and/or oil), also referred to as heavy fraction. Vessel 1 isprovided with a connecting stub 2 for infeed of the gas/liquid mixture,a connecting stub for a liquid discharge conduit 4 for discharge ofliquid and a connecting stub 5 for discharge of substantially gas.

Via a pre-treatment unit 3 the introduced gas/liquid mixture is guidedto a lower compartment A of vessel 1. In the shown embodiment thepre-treatment unit 3 consists of a tube provided on the underside withan opening. This opening ensures that the supplied mixture is drivenunder high pressure in the direction of the bottom of the vessel. At thebottom of vessel 1 the liquid F₁ is collected and drained via a conduit4.

In a preferred embodiment (not shown) the pre-treatment unit is formedby a device provided with a number of curved blades which uniformlyabsorb the moment of the incoming gas/liquid flow. This device is knownin the field as the “schoepentoeter” (vane feed inlet). The bladessubsequently guide the gas/liquid flow laterally into the lowercompartment of the separating vessel. As a result of this controlledentry of the gas/liquid mixture a first part of the liquid will alreadybe separated and accumulate at the bottom of vessel 1. It is howeveradvantageous to embody the pre-treatment unit in the form of one or moreinlet cyclones. Such inlet cyclones are for instance known from thepublication WO 00/74815 A2 of the present applicant, the content ofwhich should be deemed as interpolated herein. In such cyclones theinflowing mixture is set into a rotating movement, whereby the heavyfraction, in which a relatively large amount of liquid is present, isflung against the outer wall of the cyclone and comes to lie at thebottom of separating vessel 1, and whereby the light fraction, in whicha relatively large amount of gas is present, is guided upward.

The separated part of the mixture which, although it contains lessliquid than the mixture supplied from outside, still has a considerableliquid content, is then guided at high speed through a layer of tightlypacked wires, also referred to as a wire mesh 10. This mesh 10 isdisposed horizontally in vessel 1 and thereby forms a separation betweenlower compartment A and upper compartment B of vessel 1.

A detailed top view of such a wire mesh 10 is shown in FIG. 2. Providedin the upper surface of mesh 10 is a collecting trough or collectingreservoir 11 in which liquid F₁ can be collected. Via two dischargeconduits 12 extending below the level of liquid F₁ at the bottom of thelower compartment, the collected liquid F₁ can be carried to the lowercompartment A of vessel 1, where the liquid can be drained via dischargeconduit 4.

The mesh according to the invention is designed to function in a floodedcondition This means that the velocity of the supplied mixture is sohigh that the liquid from the mixture breaks through the mesh and formsa bubbling mass above the mesh surface.

The breaking of the liquid through the mesh is dependent on a number ofparameters, such as the velocity of the supplied gas, the density of thegas and the density of the liquid. A characteristic measure known in thefield for breaking through a mesh is the K-value. The K-value isproportional to the gas velocity multiplied by the root of the ratio ofthe gas density to the density difference of gas-liquid.

At an increasing velocity of the mixture the liquid saturating planewill displace increasingly further upward (for instance from a positionv₂ to a position v₂, FIG. 3). The mesh is disposed horizontally andmoreover has a practically constant thickness d (between 100 and 200 mm)to ensure that the saturating plane extends substantially parallel tothe outer surface of the mesh. Once the saturating plane has reached theupper surface of the mesh and a breakthrough occurs, the breakthroughwill thereby take place uniformly over the whole upper surface. In thecase of a varying thickness or a non-horizontal arrangement,breakthrough would only occur on a limited part of the upper surface,while no breakthrough would take place at the position of the remainingpart. Such a partial breakthrough results in an irregular distributionof the mixture downstream of the mesh, which has a negative effect onthe separation efficiency of the separator disposed downstream.

It has been found that if the K-value is greater than about 0.1,breakthrough will occur. In order to now prevent too much liquidaccumulating above the mesh surface and the distribution of the mixturedisplacing upward downstream still being adversely affected, the liquidis collected via the above mentioned collecting reservoir 11 and carriedvia discharge conduits 32 to the lower compartment A.

Shown in FIG. 4 is another preferred embodiment in which the collectingtrough 11′ is placed centrally in the middle of the mesh pad, and onlyone discharge conduit 12′ is provided for draining liquid. For anoptimal agglomerating action of mesh pad 10 the cross-sectional surfacearea of collecting trough 11, 11′ relative to the cross-sectionalsurface area of mesh pad 10 should be between 5% and 25%, and preferablyaround 15%.

The mixture which is displaced further upward, in which mixture arelatively large amount of gas and liquid is present in relatively largedroplets, is further separated by a number of cyclone separators. Anumber of boxes 13 is arranged for this purpose in upper compartment Bof vessel 1 (FIG. 1) Provided downstream thereof is the connecting stub5 for discharging the gas which is dried to a considerable extent. Boxes13 are each separately or jointly provided with a downcomer 6 which isin communication with liquid F₁ at the bottom of the vessel for drainingliquid from each of the boxes.

Eight cyclones 14 are arranged in each of the boxes 13. In aparticularly effective embodiment these are axial recycle cyclones. FIG.5, which is practically the same as FIG. 2 of the aforementioneddocument WO 00/25931, shows a box 13 provided with a number of cyclones.A cyclone comprises a cylindrical wall 15 which forms on the undersidean inlet for the gas/liquid mixture and has an outflow opening 16 on theupper side thereof. Roughly centrally in the space enclosed by thecylindrical wall is placed a so-called swirl element 17 which isprovided with blades 18 for setting the mixture into rotating movement.A part of the mixture is flung outward by this rotating movement andtransported via an interspace 19 to a recycle conduit 20. Recycleconduit 20 extends through swirl-element 17. Further connected to thespace between wall 15 and the wall of the box is a conduit 21 fordraining liquid, which conduit debouches onto a ring line 22 into whichdrain conduits of other cyclones debouch on one side and downcomer 6 isconnected on the other side to the space at the bottom of the vessel forcollecting liquid F. The separation efficiency of the type or axialrecycle cyclone in combination with mesh 10 described herein has beenfound to be particularly high, which enables among other things acompact embodiment of the installation.

The present invention is not limited to the above described embodimentsthereof; the rights sought are defined by the following claims, withinthe scope of which many modifications can be envisaged.

1. Device for treating a gas/liquid mixture, comprising: an uprightvessel with a lower and upper compartment; an inlet for admitting theflow of mixture into the lower compartment; an agglomerating unit placedbetween the lower and upper compartment for enlarging the liquiddroplets in the mixture; a separator arranged in the upper compartmentdownstream of the agglomerating unit for further separating the mixtureinto a substantially liquid-containing mixture part and a substantiallygas-containing mixture part; a lower outlet for discharging thesubstantially liquid-containing mixture part from the lower compartment;an upper outlet for discharging the substantially gas-containing mixturepart from the upper compartment; collecting means for collecting in ordownstream the agglomerating unit the liquid droplets which have brokenthrough the agglomerating unit; and recycling means for recycling thecollected liquid from the collecting means to the lower compartment. 2.Device for treating a gas/liquid mixture, comprising: an upright vesselwith a lower and upper compartment; an inlet for admitting the flow ofmixture into the lower compartment; an agglomerating unit placed betweenthe lower and upper compartment for enlarging the liquid droplets in themixture; a lower outlet for discharging the substantiallyliquid-containing mixture part from the lower compartment; an upperoutlet for discharging the substantially gas-containing mixture partfrom the upper compartment; collecting means for collecting on thedownstream side of the agglomerating unit liquid droplets which havebroken through the agglomerating unit; and recycling means for recyclingthe collected liquid to the lower compartment from the collecting means.3. Device according to the claim 1, wherein the agglomerating unitcomprises a wire mesh.
 4. Device as claimed in claim 1, wherein thecollecting means comprise at least one collecting reservoir extending inthe agglomerating unit for collecting the broken-through liquid therein,and wherein recycling means comprise a discharge conduit extending fromthe collecting reservoir to below the level of the liquid collected inthe lower compartment.
 5. Device as claimed in claim 1, wherein thecollecting means are arranged over substantially 15% of thecross-section of the vessel.
 6. Device as claimed in claim 3, whereinthe mesh is embodied to allow the supplied liquid to break through froma minimum K-value of about 0.1.
 7. Device as claimed in claim 1, whereinthe agglomerating unit extends substantially horizontally.
 8. Device asclaimed in claim 1, wherein the agglomerating unit extends oversubstantially the whole cross-section of the upright column.
 9. Deviceas claimed in claim 1, wherein the thickness of the agglomerating unitis substantially constant.
 10. Device as claimed in claim 1, wherein theseparator comprises one or more cyclone separators.
 11. Device asclaimed in claim 1, wherein the separator comprises one or more axialrecycle cyclones, the liquid discharge of which extends from theseparator to below the liquid level in the lower compartment.
 12. Deviceas claimed in claim 1, wherein the inlet is connected to a pre-treatmentunit for carrying out a first liquid/gas separation, which pre-treatmentunit comprises an inlet cyclone separator arranged in the lowercompartment.
 13. Method for treating a gas/liquid mixture in an uprightvessel with a lower and upper compartment, comprising: feeding thegas/liquid flow into the lower compartment of the column; guiding thegas/liquid flow through an agglomerating unity arranged between thelower and upper compartment at high speed such that liquid breaksthrough to a position beyond the downstream surface of the agglomeratingunit; collecting the broken-through liquid; recycling the collectedliquid to the lower compartment; guiding the mixture through a separatorin the upper compartment for further separating the mixture into asubstantially liquid-containing mixture part and a substantiallygas-containing mixture part; and discharging the mixture from the lowercompartment and discharging the mixture from the upper compartment. 14.Method for treating a gas/liquid mixture in an upright vessel with alower and upper compartment, comprising: feeding the gas/liquid flowinto the lower compartment of the column; guiding the gas/liquid flowthrough an agglomerating unit arranged between the lower and uppercompartment at high speed such that liquid breaks through to a positionbeyond the downstream surface of the agglomerating unit; collecting thebroken-through liquid; recycling the collected liquid to the lowercompartment; discharging the mixture from the lower compartment; anddischarging the mixture from the upper compartment.
 15. Method asclaimed in claim 13, wherein the K-value of the supplied mixture amountsto at least 0.1.
 16. Method as claimed in claim 13, further comprisingfurther separating the mixture in the upper compartment by guiding themixture through one or more cyclone separators and carrying theseparated liquid part to the lower compartment and the separated gaspart to the upper outlet.
 17. Method as claimed in claim 13, furthercomprising pre-treating the introduced gas/liquid flow for separating apart of the liquid from the gas/liquid mixture, wherein the pretreatmentcomprises of guiding the introduced gas/liquid mixture through one ormore inlet cyclones.
 18. Method as claimed in claim 13, to be performedin a device.
 19. Device as claimed in claim 1, wherein the liquidcontains at least one of oil and/or water.
 20. Device according to theclaim 2, wherein the agglomerating unit comprises a wire mesh. 21.Device as claimed in claim 2, wherein the collecting means comprise atleast one collecting reservoir extending in the agglomerating unit forcollecting the broken-through liquid therein, and wherein recyclingmeans comprise a discharge conduit extending from the collectingreservoir to below the level of the liquid collected in the lowercompartment.
 22. Device as claimed in claim 2, wherein the collectingmeans are arranged over substantially 15% of the cross-section of thevessel.
 23. Device as claimed in claim 4, wherein the mesh is embodiedto allow the supplied liquid to break through from a minimum K-value ofabout 0.1.
 24. Device as claimed in claim 5, wherein the mesh isembodied to allow the supplied liquid to break through from a minimumK-value of about 0.1.
 25. Device as claimed in claim 10, wherein theseparator comprises one or more axial recycle cyclones, the liquiddischarge of which extends from the separator to below the liquid levelin the lower compartment.
 26. Method as claimed in claim 14, wherein theK-value of the supplied mixture amounts to at least 0.1.
 27. Method asclaimed in claim 14, further comprising further separating the mixturein the upper compartment by guiding the mixture through one or morecyclone separators and carrying the separated liquid part to the lowercompartment and the separated gas part to the upper outlet.
 28. Methodas claimed in claim 14, further comprising pre-treating the introducedgas/liquid flow for separating a part of the liquid from the gas/liquidmixture, wherein the pretreatment comprises of guiding the introducedgas/liquid mixture through one or more inlet cyclones.
 29. Method asclaimed in claim 14, to be performed in a device.
 30. Device as claimedin claim 2, wherein the liquid contains at least one of oil and water.31. Method as claimed in claim 13, wherein the liquid contains at leastone of oil and water.
 32. Method as claimed in claim 14, wherein theliquid contains at least one of oil and water.